I don't think so.
Routes have to be re-calculated during import on the Map that is currently Active.
You could possibly split the route into sections "road sections" and "off road sections".
Then You would end up with a number of routes that you would have to select as you go along.
Waypoints can be used to force the nuvi to calculate the route the way you want it.
However if there is no road then it's a problem.